1. Field of the Invention
The present invention is related generally to preparation of thin film compounds and more particularly to preparing thin film compounds of Cu(In,Ga)Se.sub.2 in semiconductor devices.
2. Description of the Prior Art
Thin films of copper-indium-diselenide (CuInSe.sub.2), copper-gallium-diselenide (CuGaSe.sub.2), and copper-indium-gallium-diselenide (CuIn.sub.1-x Ga.sub.x Se.sub.2), all of which are sometimes generically referred to as Cu(In,Ga)Se.sub.2, have become the subject of considerable interest and study for semiconductor devices in recent years. They are of particular interest for photovoltaic device or solar cell absorber applications because of solar energy to electrical energy conversion efficiencies that have been shown to exceed 15% in active areas and to approach 14% in total areas, which is quite high for current state-of-the-art solar cell technologies. It is generally believed by persons skilled in this art that the best electronic device properties, thus the best conversion efficiencies, are obtained when the mole percent of copper is about equal to the mole percent of the indium, the gallium, or the combination of the indium and gallium in the Cu(In,Ga)Se.sub.2 compound or alloy. The selenium content will not generally be important to the electronic properties of the semiconductor if the growth conditions supply sufficient selenium so that it comprises about 50 at. %) of the Cu(In,Ga)Se.sub.2 compound to form the desired crystal lattice structures. Sulfur can also be, and sometimes is, substituted for the selenium, so the compound is sometimes referred to even more generically as Cu(In,Ga)(S,Se).sub.2 to comprise all of those possible combinations.
Although the growth of single crystal CuInSe.sub.2 has been studied, such as in the U.S. Pat. No. 4,652,332, issued to T. Ciszek, the use of polycrystalline thin films is really more practical. Sputter depositing a ternary single phase CuInSe.sub.2 layer, including the ability to determine the properties of the thin film, such as multilayer structures, by varying the sputter process parameters, is described by U.S. Pat. No. 4,818,357, issued to Case et al. However, the two fabrication methods of choice are: (1) Physical vapor deposition of the constituent elements, exemplified by the process disclosed in U.S. Pat. No. 5,141,564, issued to Chen et al., which is generally used as a research tool and (2) selenization of Cu/In metal precursors by either H.sub.2 Se gas or Se vapor. The selenization technology generally exemplified by the processes described in U.S. Pat. No. 4,798,660, issued to Ermer et al., U.S. Pat. No. 4,915,745, issued to Pollock et al., and the U.S. Pat. No. 5,045,409, issued to Eberspacher et al., is currently favored for manufacturing processes. However, thin films produced by the selenization processes usually suffer from macroscopic spacial nonuniformities that degrade performance and yield, and reproducible consistent quality from run to run is difficult to obtain and unpredictable.